This section is intended to provide a background to the various embodiments of the technology that are described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by its inclusion in this section.
Detailed descriptions of radio communication networks and systems can be found in literature, such as in Technical Specifications published by, e.g., the 3rd Generation Partnership Project (3GPP). 3GPP Long Term Evolution (LTE) is the fourth-generation radio communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a user equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
In 3GPP Release 10, the LTE random access procedure is a four-step procedure. The LTE random access procedure is used for initial access when establishing a radio link (e.g., moving the UE from RRC_IDLE to RRC_CONNECTED state), to re-establish a radio link after radio-link failure, to establish uplink (UL) synchronization, or as a scheduling request if no dedicated scheduling-request resources have been configured on the Physical Uplink Control Channel (PUCCH). The sequence of messages exchanged between the UE and the eNB during the random access procedure is schematically illustrated in FIG. 1, and further described below:                1. Generally speaking, the first step in the random-access procedure is the transmission of a random-access preamble on the Physical Random-Access Channel (PRACH). As part of the first step of the random-access procedure, the UE may randomly select one preamble to transmit, out of one of the two subsets defined for contention-based access. Subsets of preamble configurations within a cell can be seen in FIG. 2. Which subset to select the preamble from, can be given by the amount of data the UE would like to transmit on the Uplink Shared Channel (UL-SCH) in the third random access step. Which subset to select the preamble from, can e.g., also be given by the amount of data the UE is capable of transmitting on the UL-SCH in the third random access step (e.g., due to power limitations at the UE). Typically, subset 0 is selected by UEs with the aim of transmitting a limited amount of information, whilst preamble subset 1 is selected from UEs having assessed the potential to transmit higher payload within the third message of the random access procedure. The time/frequency resource to be used for this kind of transmissions (as illustrated in FIG. 3) can be given by the common PRACH configuration of the radio cell, which can be further limited by an optional, UE-specific mask, which may be limiting the available PRACH opportunities for the given UE (see e.g. 3GPP TS 36.321 v.10.0.0 and 3GPP TS 36.331 v.10.3.0 for further details).        2. The second step of the random-access procedure is the transmission of the Random Access Response. The eNB transmits a message on the Downlink Shared Channel (DL-SCH), the message comprising the index of the random-access preamble sequences the eNB detected and for which the response is valid, the timing correction (i.e. timing advance) calculated by the random-access preamble receiver, a scheduling grant, as well as a temporary identity (TC-RNTI, i.e. Temporary Cellular Radio Network Temporary Identifier) used for further communication between the UE and eNB. A UE which does not receive any Random Access Response in response to its initial random access preamble transmission of step 1 above within a pre-defined time window, i.e. time period, will generally consider the random access attempt as failed. If so, the UE will generally repeat its random access preamble transmission (possibly with higher transmit power) up to a number of maximum attempts (e.g. four times), before determining the entire random-access procedure as failed.        3. A purpose of the third step of the random access procedure is to assign a unique identity to the UE within the cell (C-RNTI, i.e. Cell Radio Network Temporary Identifier). In this step, the UE can transmit the necessary information to the eNB using the UL-SCH resources assigned to the UE in the Random Access Response. This message can thus allow the UE to adjust the grant size and modulation scheme as well as allowing for HARQ (i.e. Hybrid Automatic Repeat Request (ARQ)) with soft combining for the uplink message.        4. The fourth and final step of the random-access procedure is generally a downlink (DL) message for contention resolution. Based on the contention resolution message, each UE receiving the downlink message will compare the identity in the received message with identity transmitted in the third step. Only a UE that observes a match between the identity received in the fourth step and the identity transmitted as part of the third step will determine the random-access procedure to be successful. Otherwise, the UE will generally restart the random access procedure.        
A more detailed description of the LTE random access procedure can be found in literature, such as in the reference book 4G LTE/LTE-Advanced for Mobile Broadband by Erik Dahlman, Stefan Parkvall and Johan Sköld, Academic Press, 2011, ISBN:978-0-12-385489-6; see particularly chapter 14.3 “RANDOM ACCESS”.
FIG. 4 illustrates the time-domain structure of PRACH preamble formats 0 to 3 as specified by 3GPP LTE. It should be appreciated that the PRACH preamble to be transmitted during the random access procedure can be one out of five different formats. In preamble format 0 and in preamble format 1, the preamble (excluding the cyclic prefix) is 800 μs long. Preamble format 0 and 1 prefix a cyclic prefix of 103 μs and 684 μs, respectively. The cyclic prefix should preferably cover the uncertainty in round trip time (plus maximum delay spread) in the radio cell. Preamble format 0 can thus be used in cells of up to approximately 15 km (100 μs roundtrip time) and preamble format 1 for radio cells of a radius of 100 km (667 μs roundtrip time). The main part of the cyclic prefix is in both cases 800 μs. That is, the path loss that both formats can sustain is generally the same. 3GPP LTE also specifies preamble formats 2 and 3, respectively, where the main part of the preamble is 1600 μs and comprises the twice repeated (excluding cyclic prefix) preamble formats 0 or 1. The cyclic prefix for preamble formats 2 and 3 is 203 μs and 684 μs, respectively. Due to the twice as long main part of the preamble (compared to preamble formats 0 and 1) preamble formats 2 and 3 can operate approximately at up to 3 dB higher path loss. Reference is made to FIG. 4 for a graphical illustration of the different preambles. Preamble formats 0 to 3 all span a bandwidth of approximately 1 MHz. In addition to the preamble formats 0 to 3 shown in FIG. 4, 3GPP LTE also defines a very short Format 4 which will not be further detailed herein. Typically, within one cell, one format is used. A more detailed description of the different preamble formats can be found in literature, such as in the reference book 4G LTE/LTE-Advanced for Mobile Broadband by Erik Dahlman, Stefan Parkvall and Johan Sköld, Academic Press, 2011, ISBN:978-0-12-385489-6, see particularly chapter 14.3.1.1 “PRACH Time-Frequency Resources”.
After having successfully received the PRACH preamble, the eNB is generally aware that a PRACH preamble has been transmitted. Furthermore, the eNB is capable of deriving the time-of-arrival of the received PRACH signal and can thus calculate a timing-advance value that is needed for subsequent UL synchronization.
It should be appreciated that making the eNB aware that a PRACH preamble has been transmitted generally requires a certain quantity of energy contained in the PRACH signal. That is, the transmit power and time duration of the PRACH signal are important. Deriving the time-of-arrival of the received PRACH signal generally requires, in addition, a certain bandwidth to be able to estimate the time of arrival accurately. In 3GPP LTE this bandwidth has been determined to be around 1 MHz (Megahertz).
The inventors of the herein described technology have realized that in order to make the PRACH sustain higher path loss the simplest solution would be to just repeat the PRACH signal. As an example, if the PRACH should be able to operate at a 20 dB (decibel) higher path loss relative to the preamble format 0 a new PRACH preamble format has to be specified that repeats preamble format 0 (excluding the cyclic prefix) approximately 100 times. However, given that PRACH spans 1 MHz in frequency and approximately 1 millisecond (ms) in time, the reserved resources (i.e. 1 MHz×100 ms) would become unreasonable large. Furthermore, since the UEs that will need to use an extended PRACH signal duration are generally devices that are power-limited, the UEs do not gain much by using a wide-band PRACH signal. The available transmission power of the UE would be spread over a larger bandwidth since the power cannot generally be increased if it is already set to the maximum value the UE can support.